A fundamental problem in electrical grids is that the same connections that give a network its functionality can promote the spread of failures that would otherwise remain confined. Understanding the resulting cascading failures has been hindered by the lack of realistic large-scale modeling that can account for variable system conditions. In this presentation, I will discuss our full-scale modeling of the U.S.-South Canada power-grid network, which allows us to characterize the set of network components that are vulnerable to cascading failures. I will show that the vulnerable set consists of a small but topologically central portion of the network and that large cascades are disproportionately more likely to be triggered by initial failures close to this set. These results elucidate aspects of the origins and causes of cascading failures, and point to opportunities for failure mitigation in connection with grid design and operation.